Scientists and engineers often employ geophysical surveys for exploration and engineering projects. Geophysical surveys can provide information about underground structures, including formation boundaries, rock types, and the presence or absence of fluid reservoirs. Such information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons and ores. Oil and gas companies in particular often invest in extensive seismic and electromagnetic surveys to select sites for exploratory wells.
Geophysical surveys can be performed on land or in water using active seismic sources such as air guns, vibrator units, or explosives to generate seismic waves. Receivers such as hydrophones or geophones detect reflections of such waves from subsurface structures. The generating and receiving process is repeated with different source positions and optionally with different receiver positions. The arrangement of sources and receivers may be customized to achieve adequate coverage of the region of interest while facilitating processing of the acquired seismic data. The acquired seismic data is recorded and processed to provide a seismic image that may be used to identify subterranean features of interest.
The survey process may be periodically repeated to enable monitoring of the subsurface over time, e.g., as hydrocarbons are produced from a reservoir. A four dimensional, or 4D, seismic survey includes performing three-dimensional seismic surveys over time. When such repetition occurs, the initial survey may be termed the “baseline” survey, and subsequent surveys that are taken at later times may be termed “monitor” surveys. Monitoring is achieved by comparing the baseline survey to each of the monitor surveys, e.g., by differencing the seismic images to highlight those areas where changes have occurred over the time intervals (typically years or decades) between surveys.
However, such comparisons may face unexpected obstacles. For example, the extracted reservoir fluids may be displaced by other fluids and/or pore compression, causing the seismic wave velocities to change in the region of interest. Such wave velocity changes cause apparent shifting of the seismic horizons (at least in the unmigrated seismic images or migrated images derived with an uncorrected velocity model), which, in turn, creates the appearance of substantial changes even in areas where none has occurred. One existing approach to removing such apparent horizon shifts, called “time-warping”, is susceptible to warping error, a form of over-correction that obscures the actual changes that are sought by this comparison.